3 Answers
3

Gewiss is an adjective that suggests a fixed, but not precisely mentioned (not necessarily unknown) amount of something. The amount is not necessarily large; it is typically above average or more than nothing and indicates that there is an amount (as opposed to nothing) or that the amount is higher (than expected, for example). Bestimmt is kind of similar, but emphasizes that the amount is known and fixed (and does not suggest it to be greater or less), while gewiss mostly suggests a greater amount. It depends on context whether gewiss suggests greater or smaller amounts, or neither.

From the samples:

Eine gewisse Summe means it wasn't free, but not necessarily a huge amount. It is often used to indicate that you have to pay more for better things:

However, gewiss can not only refer to amounts, but also to other things and then has the original meaning of something known, but not mentioned (which is, of course, similar to the meaning explained above). For example:

Great answer; only one thing where I can't agree: If I read "Man braucht nur eine gewisse Menge Lösung anzuwenden", then I read it as "It suffices to apply a certain amount of the solution". What I mean, for me the nur refers to braucht and not to eine gewisse Menge. I can't understand it in the sense that less than thought is needed.
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Hendrik VogtAug 19 '11 at 6:19

@Hendrik Vogt: Not sure what you mean; if you leave out the "nur" ("Man braucht eine gewisse Menge Lösung") vs. including it ("Man braucht nur eine gewisse Menge Lösung"), the first one suggests greater and the second one suggests less, at least to me.
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OregonGhostAug 19 '11 at 8:23

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To me it suggests that you only need a certain amount of solution and nothing else. (So it's not about more or less but about needing other things or not.)
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Hendrik VogtAug 19 '11 at 8:37

This appears to be a good answer, in that it precisely asserts the correspondence to English. I can therefore understand gewissen as 'an amount that is well-understood by somebody, but not specified in this conversation'. The oustanding question is, as always: is it correct?
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Tom WApr 6 '12 at 21:42